I did a routine oil and filter change a few weeks ago and unfortunately experienced a bit of a "shock".

To remove the oil filter I reach around the back of the engine from the air intake side of the engine, putting my arm down between the coolant expansion tank and the intake manifold. I use a wide-jawed set of "mole" grips to clamp onto the filter and turn it to break the seal, then remove the tool and continue by hand.

Unfortunately this time I shower of sparks erupted from behind the engine. I quickly removed my hand and the tool and saw that the tool was scorched and scored at either end where it had obviously shorted out across the +ve terminal of the starter motor or alternator and the -ve terminal of the other or another earthed part of the engine or ancillaries - I'm not familiar with the back of the engine block so not exactly sure what shorted out. Thankfully I didn't get an electrical shock or burns - I was wearing gloves and long sleeved coveralls. I've done this operation a few times before and this is the first time I've had this problem.

Pic of the tool and scorch/score marks:

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I disconnected the battery before continuing, but it took me a while to get my head around what had happened, I think due to the mental shock of how bad this could have been (perhaps unlikely but still possible because AFAIK there's no fuses for the starter motor or alternator):- I could have had an electrical shock or burns- The tool could have welded across the short, leading to it overheating, melting and eventually setting fire to the car- In parallel the battery would have been discharging so quickly it would have released loads of hydrogen, which could have ignited, especially if I'd tried to remove the negative lead from the battery terminal which would probably have sparked- If I'd tried to remove the tool by hand when hot it could have melted my glove and stuck my hand in there.... that's when my imagination really runs riot with how horrid it could be.

So, everyone, to avoid a horrible ending please disconnect the battery before removing the oil filter (or any other operation where you need to rummage around the back of the engine block).

If anyone can explain why my worst case scenario isn't possible then I'll be very grateful as it will help me sleep at night!

Cheers,Will

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You might want to have a look under the car at the connection to the starter or alternator. The alternator should have a rubber boot over the fixing nut to prevent such unwanted fireworks. Perhaps it's been removed, lost or deteriorated? Don't know about the starter as I've not looked there myself.

Well, now I've looked I can't work out how I contacted anything! Certainly not on the starter motor, although the positive terminals are uncovered and so is the earth.

On the alternator, is the positive post at the top? If so then I must have contacted this and earthed against the engine block because the other terminal is behind where the filter from the direction I was coming at it.